Computer systems, such as speech recognition systems use a microphone to capture sound.
To facilitate discussion, FIG. 1 is a schematic view of a microphone array 102 that may be used in the prior art. Such an array may provide microphones 104 mounted on a display 108. The spacing between the microphones 104 in the microphone array is an equal spacing “d” between each microphone. In this example the total distance between microphones is about 30 centimeters (cm), so that d is about 15 cm. In speech recognition systems, a microphone array may be used to increase the signal to noise ratio by adding signals from each microphone in the microphone array. A user 112 positioned in front of the microphones 104 would speak, so that the sound waves from the user 112 would reach each microphone 104 in the microphone array 102 at about the same time. The signals from each microphone 104 would then be added in a constructive manner. Background noise may be generated by a noise source 116 located off axis from the microphone array 102. Sound 120 from the noise source 116 would reach the microphones 104 at different times, so that the signals from the different microphones would not normally be added in a constructive manner. However, if the background noise from the noise source has a wavelength (λ) of d/n, where n is an integer, then the microphones 104 would be simultaneously located at the maximums of the background noise causing a constructive addition of the signals from the microphones 104 (resonance interference). Generally, the speed of sound in dry air at about 1 atmospheres is about: v=331 m/s+(0.6 m/s/C)*T. So at about 20° C., the speed of sound in air is about 343 m/s. With distance being about 15 cm., the frequencies (f) that would cause the resonance interference so that the addition of signals from the microphones 104 would be constructively added would be f=(34,300 cm/s)*n/(15 cm)=n(1,143 Hz). For some voice recognition systems it may be desirable to process sounds with frequencies between 140 to 6,500 Hz. Therefore n=1, 2, 3, 4, 5 would yield frequencies of 1,143 Hz, 2,286 Hz, 3,429 Hz, 4,572 Hz, and 5,715 Hz, which would be within the range on a voice recognition system.
FIG. 2 is another schematic view of a microphone array 202 that may be used in the prior art. Such an array may provide microphones 204 mounted on a display 208, in a manner similar to the array in FIG. 1. The spacing between the microphones 204 in the microphone array is an equal spacing “d” between each microphone. In this example the total distance between microphones is about 30 centimeters (cm), so that d is about 15 cm. However, an additional microphone 205 is added to the array 208 between two microphones 204, so that the spacing between the additional microphone 205 and the two microphones 204 is ½ d (7.5 cm). A user 212 positioned in front of the microphone array 202 would speak, so that the sound waves from the user 212 would reach each microphone 204, 205 in the microphone array 202 at about the same time. The signals from each microphone 204, 205 would then be added in a constructive manner. Background noise may be generated by a noise source 216 located off axis from the microphone array 202. Sound 220 from the noise source 216 would reach the microphones 204 at different times, so that the signals from the different microphones would not normally be added in a constructive manner. However, if the background noise from the noise source has a wavelength (λ) of (1/2)(d/n), where n is an integer, then the microphones 204, 205 would be simultaneously located at the maximums of the background noise causing a constructive addition of the signals from the microphones 204, 205 (resonance interference). Thus, the additional microphone 205 causes the wavelength to be 7.5 cm/n. Generally, the speed of sound in dry air at about 1 atmospheres is about: v=331 m/s+(0.6 m/s/C)*T. So at about 20° C., the speed of sound in air is about 343 m/s. With the extra microphone 205 spaced 7.5 cm from the other microphones 204, the frequencies (f) that would cause the resonance interference so that the addition of signals from the microphones 204, 205 would be constructively added would be f=(34,300 cm/s)*n/(7.5 cm)=n(2,286 Hz). Therefore n=1 and 2, would yield frequencies of 2,286 Hz, 4,572 Hz, which would be within the range of a voice recognition system.
To provide improved signal to noise output more microphones may be provided to the array. FIG. 3 is another schematic view of a microphone array 302 that may be used in the prior art. Such an array may provide four microphones 304 mounted on a display 308. The spacing between the microphones 304 in the microphone array is an equal spacing “d” between each microphone. In this example the total distance between microphones is about 30 centimeters (cm), so that d is about 10 cm. In speech recognition systems, a microphone array may be used to increase the signal to noise ratio by adding signals from each microphone in the microphone array. A user 312 positioned in front of the microphones 304 would speak, so that the sound waves from the user 312 would reach each microphone 304 in the microphone array 302 at about the same time. The signals from each microphone 304 would then be added in a constructive manner. Background noise may be generated by a noise source 316 located off axis from the microphone array 302. Sound 320 from the noise source 316 would reach the microphones 304 at different times, so that the signals from the different microphones would not normally be added in a constructive manner. However, if the background noise from the noise source has a wavelength (λ) of d/n, where n is an integer, then the microphones 304 would be simultaneously located at the maximums of the background noise causing a constructive addition of the signals from the microphones 304 (resonance interference). With distance being about 10 cm., the frequencies (f) that would cause the resonance interference so that the addition of signals from the microphones 304 would be constructively added would be f=(34,300 cm/s)*n/(10 cm)=n(3,430 Hz). Therefore n=1, would yield frequencies of 3,430 Hz, which would be within the range of some voice recognition systems.
It would be desirable to provide a computer system with speech recognition, with a microphone array where the frequency of resonance interference would be outside of or near the outside of the human voice range and may even be outside of a microphone sound range or even outside of the voice recognition range.